SANFORD — Seminole County will be forced to dip into its general fund for $3.5 million over the next two years to make up a shortfall in collections from the special sales tax for the jail expansion and courthouse renovations.

Latest estimates from the Florida Department of Revenue show that the county can expect to receive $13.1 million from the extra penny on 1985 sales tax collections, plus interest that has accrued on the money invested. But the total bill for construction and renovation is expected to reach about $17 million, said Penny Fleming, the county's acting budget director.

County voters in November 1984 authorized an extra cent per dollar on the state sales tax for calendar 1985 to pay for the larger jail and work on the courthouse. At the time it was estimated that the projected $13.6 million in collections would match construction costs.

However, collections were off slightly because of lower retail sales, and bids for construction came in higher than expected early last year. Expenses totalling about $100,000 also were added to the courthouse project, including extra security measures.

The county commission tentatively has decided to pay the extra expenses from the general fund budget this year and next rather than borrow the money. The federal government is paying $1 million of the extra tab to reimburse the county for federal prisoners who are housed in the jail.

This decision puts a strain on next year's budget and prompted County Administrator Ken Hooper to ask county departments to trim spending. Department heads are bringing budget requests to the commission this week. Those request average almost 8.2 percent less than this year's spending.

Commissioners next week are scheduled to consider an amendment to this year's budget to authorize the use of $1.1 million in general funds to complete the $3 million renovation at the county courthouse.

Of that total, $700,000 is coming from the federal government, which was persuaded to raise its share of the jail expansion project from $300,000 to $1 million.

Courthouse work to change old office space into eight courtrooms, add office space for judges, remove asbestos from hallway ceilings, repaint several areas, increase security and replace carpet is scheduled for completion in September.

In addition, $2.4 million is to be set aside from next year's budget to help complete work on the 480-bed addition to the jail at Five Points. It is expected to be finished in August 1987. The existing jail will be renovated afterward. By spring 1988 it will have bed space for 724 prisoners.